This invention is related to composite structures and in particular to composite structures of thermoplastic polyolefins.
Exterior auto and truck parts such as bumpers, fender extensions, wheel covers, hub caps, trim rings for wheels, lamp housings, grills, other facia components and other molded exterior parts of thermoplastic polyolefins (TPOs) such as polypropylene polyethylene are currently being used because of their resistance to permanent deformation on impact and their corrosion resistance. These parts replace metals which are easily dented and subject to corrosion and rapid deterioration by weathering. TPOs are easily and economically injection molded into auto and truck parts that are substantially lighter than currently used sheet metal parts thereby favorably affecting the weight and fuel economy of the auto or truck. Another advantage of TPOs is that more intricate designs can be made in comparison to sheet metals giving the auto makers increased freedom in design of autos.
Though less expensive then other conventional plastics used for automotive parts, TPOs have made only a limited penetration into the enormous automotive facia and vertical autobody panel market. The most critical problem perceived by the automobile manufacturers about the use of TPOs on automobiles and trucks is the difficulties encountered in painting them. TPOs have an extremely low surface energy and most conventional automotive paints do not wet the TPO's surface or adhere to the TPO. A number of techniques have been used to modify the surface of the TPO or special sealers have been used to overcome these problems but none have been completely successful.
The most common method employed to overcome the adhesion problem is to seal the TPO injection molded part with about a 5-10 micron thick layer of a sealer containing chlorinated carboxylated polyalphaolefin such as a chlorinated maleated polypropylene as shown in Folzenlogen et al U.S. Pat. No. 3,579,485 issued May 18, 1971. Another is shown in Baseden U.S. Pat. No. 4,303,697 issued Dec. 1, 1981 in which a polyolefin substrate such as polypropylene is primed with a chlorinated polypropylene containing carboxylic anhydride groups and then exposing the primer to ultraviolet radiation which improves both dry and wet adhesion of subsequently applied paints. Other techniques have been used to change the surface chemistry of the TPO substrate such as the use of plasma and arc treatments.
However, coating or treating complex shapes that contain recesses is extremely difficult in practice on the scale required for automotive production using the above methods and techniques and numerous cases of finish delamination from the TPO substrate after use on automobiles and trucks in the field have been experienced.
Another problem encountered with the painting of TPO parts is the need for a primer coat between the sealer coat or the treated TPO substrate and the topcoat finish or paint. Primers applied over metal substrates have functional uses such as providing corrosion resistance and providing a smooth surface. When primers are applied over TPO substrates, they are applied only to provide a smooth or class A surface to which the paint or topcoat finish is applied. A smooth surface on TPOs can be produced by polishing the injection mold in which they are formed. However, on removal of the injection molded part from the polished mold, the surface of the part can be damaged which then requires the use of a primer to provide a smooth surface.
Still another problem encountered with painting TPO parts, is the need for special jigs or hangers to maintain the shape of the part during the baking of the primer and the topcoat. These special hangers or jigs for each part represent a major capital expenditure and add cost and complexity to the finishing process.
To reduce air pollution, automobile and truck manufactures need to reduce the amount of painting in the manufacturing process and do not want to add additional painting steps to the process to coat TPO parts. It would be desirable to have a TPO auto or truck part that has a high quality durable and weatherable surface whose color matches the painted sheet metal used to make the auto or truck and that can attached directly in the assembly process and does not require further priming or painting. This would eliminate the costly and time consuming painting process for TPOs with the associated pollution problems.